The present invention relates generally to methods for dehalogenation of organic compounds, and more specifically, to improved methods for dechlorinating aromatic hydrocarbons electrochemically.
Halogenated organic compounds have many important commercial applications, such as in agriculture and medicine. Some halogenated compounds, in particular polychlorinated biphenyls (PCB's), have specially attractive properties, such as low melting points, low flammability, low volatility and high stability to chemical and biodegradation. As a result, there has been wide spread use of PCB's as insulating materials in electrical equipment, fire retardants, heat exchange liquids, plasticizers and many other industrial applications. However, PCB's and other halogenated organics have become health hazards. PCB's for instance, once absorbed tend to deposit and remain in fatty tissue and can accumulate to toxic levels.
Significant efforts have been made to develop effective means for eliminating these highly stable compounds from the environment. One means for recovering and destroying halogenated organics involves a multi-step extraction-dechlorination process wherein a liquid carrier contaminated with PCB's, such as oil from transformers, is treated with specially selected organic solvents which are immiscible in the liquid carrier but will extract the halogenated compounds therefrom. The solvent containing the PCB's is separated from the carrier which can then be recycled for further use. The contaminated nonaqueous solvent stream is then dehalogenated electrochemically in a cell equipped with an anode and cathode. An electric potential impressed across the anode and cathode reduces the halogenated organic compounds at the cathode. Advantages of the process include the ability to treat liquid carriers, e.g. oil, having high concentrations of PCB's while permitting reclamation of the carrier liquid. A further advantage is that it does not produce a residue which is difficult to dispose. It can also be scaled to handle smaller quantities of liquids typically found in electrical transformers. One such electrolytic dehalogenation process is disclosed in co-pending U.S. application Ser. No. 643,148, filed Aug. 28th, 1984 by Harlan J. Byker.
In conducting electrolytic dehalogenation processes cells may be equipped with membranes separating reactions taking place at the cathode and anode. The anode, for example, may be a dimensionally stable type, e.g. titanium coated with ruthenium dioxide, carbon and the like. Cathodes may be solid metal, such as zinc, lead, tin or liquid metals, such as mercury. It was found, however, that in electrolytic dehalogenation processes metal cathodes did not fully destroy PCB's down to levels of less than 1 ppm as required by government environmental regulations. In addition, metal cathodes became corroded during electrolysis resulting in a fall in current efficiency. Although mercury in many cases has been the cathode of choice, it too has shortcomings, namely low current density, potential for environmental problems and the inability to obtain high surface areas for scale up.
Carbon electrodes have also been suggested in electrolytic dehalogenation processes. For example, European application No. 27,745 discloses graphite electrodes generally in the degradation of 2,7-dichlorodibenzo-p-dioxin and PCB's. U.S. Pat. No. 4,161,435 discloses an electrochemical process for reducing the level of contaminants, including PCB's in an aqueous electrolyte containing graphite flakes. The electrodes without identifying whether they are anodes or cathodes are said to comprise stainless steel, aluminum, platinum or a platinum group metal, as such, or coated onto titanium or tantalum or a non-metallic conductor, such as carbon (graphite). The electrode is said to be any convenient form, e.g. solid or perforated bar, woven cloth or fiberous form. U.S. Pat. No. 4,443,309 discloses a process for the electrochemical detoxification of organic compounds, including halogenated hydrocarbons of 1 to 10,000 ppm. An electrode material of carbon or graphite fibers can be used.
Notwithstanding their generally accepted use in electrochemical dehalogenation reactions, it was discovered that many carbon electrodes undergo degradation and have short useful lives and are basically unreliable. In the case of carbon anodes, it is known that they undergo oxidation. But in the case of cathodes the reasons for their deterioration are less clear. One theory may be, that possibly radical species form in solution, such as through the reduction of oxygen to peroxide, which can attack the carbon. One other possible theory is that radical species from the electrolyte components also attack the cathode. A further theory in the case of graphite is the intercalation process wherein ions or molecules in solution migrate between basal planes of the carbon to cause fracturing.
Accordingly, there is need for a more reliable process for the electrochemical destruction of halogenated organic compounds to very low levels which will meet government regulations, including one which is capable of destroying even low levels of halogenated aromatic compounds, i.e. 100 ppm or less in solution, at high conversion efficiencies.